Assuming that your operator lets you assign whatever IP address you like (*after* having established the GPRS session) and that they use the IP address for billing purposes, *then* you might be onto something.

But if the above is true, then Indian operators suck big time.

I don't know what you mean by "port number" in this context. You probably don't know either, so this is more a rhetorical question.

Lastly, if "most of people" actually do what you think and it works, then why ask here instead of asking "most of people".